Retardation of rubber vulcanization

ABSTRACT

Cyclo-2,5-diene derivatives particularly 4-imino-cyclohexa-2,5dien-1 ones and cyclohexa-2,5-dien-1,4-di-imines are used as prevulcanization inhibitors for natural and unsaturated rubbers compounded with sulphur vulcanizing agents and thiazole vulcanization accelerators. It is claimed that the inhibitors do not reduce the rate of vulcanization and do not interfere with pphenylenediamine antidegradants; also that their reaction products exert an antidegradant effect in the vulcanized rubber.

United States Patent Cain et al.

RETARDATION OF RUBBER VULCANIZATION lnventors: Maurice Edward Cain; Geoffrey Thomas Knight; Peter McHugh Lewis; Ian Richard Gelling, all of London, England Assignee: Natural Rubber Producers Research Association, London, England Filed: Aug. 23, 1973 Appl. No.: 390,740

Foreign Application Priority Data Aug. 23, I972 United Kingdom 39354/72 US. Cl. 260/780; 260/459 QB; 260/459 R; 260/944; 260/96 R Int. Cl C08c 11/40; C08d 1/38 Field of Search 260/459, 94.4, 96 R; 252/401, 402, 403, 405

[ May 6, 1975 [56] References Cited UNITED STATES PATENTS 2,170,191 8/1939 Fisher 260/806 3,032,520 5/1962 260/459 R 3,035,014 5/1962 Popoff 260/45) R Primary Examiner-V. P. Hoke Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [57] ABSTRACT Cycle-2,5-diene derivatives particularly 4-imino-cyclohexa-2,S-dien-l ones and cyclohexa-2,5- dien-l,4-di-imines are used as pre-vulcanization inhibitors for natural and unsaturated rubbers compounded with sulphur vulcanizing agents and thiazole vulcanization accelerators. It is claimed that the inhibitors do not reduce the rate of vulcanization and do not interfere with p-phenylenediamine antidegradants; also that their reaction products exert an antidegradant effeet in the vulcanized rubber.

7 Claims, No Drawings RETARDATION OF RUBBER VULCANIZATION Many compounding ingredients, including chemical anti-ozonants of the substituted p-phenylenediamine type are detrimental to the scorch safety of rubber containing free sulphur, even in the presence of modern delayed action accelerators of the sulphenamide type, thus necessitating the use of vulcanization retarders. The latter traditionally include acidic materials which suffer from the drawbacks that they reduce the rate of vulcanization and interfere with the basic antidegradants.

Crosslink formation in a thiazole derivative accelerated sulphur vulcanizing system is accompanied by the production of 2-mercaptobenzothiazole (MBT) which acts as a catalyst for the vulcanization reaction. Recently a new class of retardeers, pre-vulcanization inhibitors" (PVI), has been developed which improve the processing safety of rubber mixes compounded with a thiazole derivative as accelerator and sulphur, without affecting the rate of vulcanization. These compounds, alkylthioamines, react with MBT (R. l. Leib, A. B. Sullivan and C. D. Trivette Jr. Rubber Chem. and Tech., 1970, l 188.) and hence halt its catalytic action until all the PVl has been consumed, thus considerably improving the processing safety of the rubber mix (K. S. Baistany and A. B Sullivan, Tetrahedron Letters, 1970, 3547.). The PVl/MBT reaction products do not however improve the antidegradant properties of the vulcanizates.

The object of the present invention is to provide improved scorch or pre-vulcanization resistance to mixes without excessively reducing the rate of vulcanization.

The invention accordingly provides a method of inhibiting the premature vulcanization of a rubber vulcanizable by heating with a sulphur vulcanizing agent and a thiazole vulcanization accelerator, which method comprises incorporating into the rubber, in addition to the sulphur vulcanizing agent and the thiazole vulcanization accelerator at cyclohexa-2,5-diene derivative having the formula where X is O or NR and R and R may be the same or different and are inert groups.

in another aspect, the invention further provides a vulcanizable composition comprising a sulphur-vulcanizable rubber, a sulphur vulcanizing agent, a thiazole vulcanization accelerator, and, as a premature vulcanization inhibitor, a cyclohexa-2,5-diene as defined above.

The rubber may be a natural rubber or a synthetic diene rubber, i.e., any rubber having an unsaturated carbon chain.

The sulphur vulcanizing agent may be free elemental sulphur or may be partly or wholly derived from sul phur donors, and is generally used in an amount of 0.5 to 3.0 parts per parts by weight of dry rubber Thiazole accelerators are well known in the rubber compounding field, where they are generally used in amounts of from 0.l to 2.0 pphr. Conventional thiazole accelerators and their derivatives include.

N-cyclohexyl-2-benzothiazolesulphenamide N-tert-butyl-2-benzothiazolesulphenamide 2-(morpholinothio) benzothiazole benzothiazyl disulphide Z-mercapto-benzothiazole 2-benzothiazolyl diethyldithiocarbamate N,N-di-isopropyl-2-benzothiazolesulphenamide N,N-diethyl-2-benzothiazolesulphenamide N,N-dicyclohexyl-2-benzothiazolesulphenamide 2-(2,-dimethylmorpholinothio) benzothiazole.

The vulcanizable rubber mix may include other ingredients, whose natures and amounts are well known in the rubber compounding field. Thus, zinc, generally in the form of zinc oxide in an amount of from 2 to 25 pphr, and stearic acid or other fatty rubber-soluble acid in an amount of from 1 to 10 pphr, will usually be present as secondary activators. The rubber may be filled, e.g., with furnace or other carbon black, or unfilled. Other compounding ingredients, accelerators, antidegradants, processing aids, fillers and extenders may be present, in accordance with the knowledge in the art.

The cyclohexa-2,5-diene derivatives are generally used in amounts offrom 0.1 to 10 pphr. Below 0.5 pphr the inhibiting effects are likely to be small. Increasing the amount of inhibitor above 5.0 pphr does not very much increase the inhibiting effect. In the formula above, when X is NR (which is preferred), R and R may be the same or different, and are both inert groups, e.g. hydrocarbon groups, such as alkyl or cycloalkyl groups preferably containing up to 10 carbon atoms, or aryl groups, preferably monocyclic aryl groups.

The cyclohexa-2,5-diene derivatives act in the same way as the alkyl thioamine pre-vulcanization inhibitors is. they react with MBT and hence stop vulcanization until all the cyclohexa-2,5-diene derivatives have been consumed. Thus these new pre-vulcanization inhibitors are only effective in a thiazole derivative accelerated system. In the absence of an accelerator the cyclohexa- 2,5-diene derivatives are themselves accelerators, considerably increasing the rate of sulphur vulcanization (Fisher US. Pat. No. 2170191 (1934)).

An advantage of the present invention is that the major products from the cyclohexa-2,5-diene/MBT reaction are antidegradants thus improving the properties of the vulcanizates. The products obtained from a typical cyclohexa-2,5-diene derivative and MBT are shown in scheme 1.

Schema I N l/l N C Rubbers compounded as described above can be processed, e.g., by calendering or extrusion or forming into a desired shape prior to vulcanization, with reduced risk of scorch. The invention also enables the processing safety of scorched or partially scorched rubber mixes, i.e., those whose cumulative heat history during processing has rendered them unsuitable for recycling or further processing. to be recovered so that they may be used safely.

The retarders may be used in the presence of substituted p-phenylenediamine antiozonants without loss of ozone resistance. Partial or total replacement of these antiozonants by the retarders further improves scorch safety while ozone resistance remains high.

The inclusion of these retarders to rubber mixes does not adversely affect the initial physical properties of the final vulcanizates.

The invention is illustrated, but not limited. by the following examples.

EXAMPLE I The improvements in processing safety, as measured by the Mooney Scorch Test are shown in Table l. The vulcanizates being prepared according to the following formulations:

Formulation Ia lh lc Id lc lf lg Ih Ribbed Smoked Sheet I00 100 100 100 100 [O0 100 100 Zinc oxide 5 5 5 5 5 5 5 5 Stcaric acid 2 2 2 2 2 2 2 2 Process oil 5 5 5 5 5 5 5 5 HAF black 45 45 45 45 45 45 45 N-CycIohexyl-Z-benzothiazoylsulphenamide 0.5 0.5 1.8 0.5

(CBS) thiazoylsulphcnamide 0.5

2-( Morpholinothio) benzothiazole 0.5

Z-Mercaptobenzothiazole 0.5

Benzothiazyl disulphide 0.75

Zinc diethyldithio carbamate 0.2

Diphenyl guanidine 0,2

Sulphur 2.5 2.5 2.5 2.5 1.2 2.0 2.0 2.5

Additive Varies Table l Mooney Scorch 120C. Formulation Additive Conc' Minimum t, t

(pphr) Viscosity (min. (min) la None 26 20 45 la N-Nitrosodiphenylamine rctarder l 30.5 10 la N-Cyclohexylthiophthalmide 0.1 26.5 3L5 4.5 la N-Cyclohcxylthiophthalmidc 0.2 29 40 7.0 In N-Phenyl-4-iminocyclohexa-lj'dicnc- I -one 1.0 29 27.5 12.7 la N-( l .3-Dimcthylbutyl )-4-iminncyclohexa-2 5dicn- 1 one [.0 32 27 9.8 la N-lsopropyl-N '-phcnylcyclohcxa-2.S-dienc-l ,4-diiminc 2.5 I3 36 6 la N-Cyclohcxyl-N'-phenylcy|:lohcxa-2,5-diene-1,4-diimine [.0 20 30.5 6 la 2.5 l 1 35.5 7.5 la 5.0 l4 37 6.5 lu N-( l-McthylheptyH-N -phcnyl-cyclohcxu2.5-dicnel .4-diimine 2.5 14 36 7.0 lu N-(1,3-Dimethylbutyl)N"phcnyIcyCIohexu-Z,S-dicnc-1.4-diimine 2.5 9 38 65 la N.N'-DipheriylcyclohcXa-2.5-dicnc-l ,4-diiminc 1.0 18 3| 9 la N.N'-Di[l-ethyl-3-methylpcntyl)cyclohcxa-LS-dicne1.4diiminc 1.0 17 3| 5 la N,N-Di( l-cthyl-3-methylpcntyl)cyclohcxa-2,5-dicnc-l,4 diiminc la Table 1 --Continued Mooney Scorch 120C. Formulation Additive Conc Minimum l, 1

(pphr) Viscosity (min.) (min.

N-Cyclohexyl-N "phenylcyclohexa-2,S diQnc- 1 ,4-diiminc 1.5

la N.N'-Di( lelhyl 3-melhylpentyl)cyclohexa-Lidienc-l,4-diiminc 1.5 19 36.5 6.0 lb None 36 28.5 7 lb N-Cyclohcxyl-N'-phenylcyclohexa-2.S-dicne-1,4-diimine 2.0 19 39 lo None 37 25 l 1.5 Ic N-CyclohcXyl-N '-phenylcyclohexa-2.S-diencl .4-diimine 2.0 19 36.5 16 1d None 14 1.0 [L1 N.N'-Di( 1-cthyl-3-mcthylpentyl)cyclohcxa-2,5-dienc-l .4-diiminc 2.5 28 3 1c None 43 29.5 3 le N-(1.3-DimcthylhutyH-N'-phenylcyclohcxa-2.5-dienc-l,4-diiminc 2.5 23 36 4 If None 52 l l 3 lf N-Cyclohcxyl-N'-phenylcyclohcxa-2,S-diene-1.4-diiminc 0.5 37 13.5 5 lg None 31 3 lg N-Cyclohexyl-N'-phcnylcyclohcxa-Zj-dieno1,4-diimine 3.0 25 25 9 1h None 27 19 4 1h N-CyclohcxyLN "phcnylcyclohcXflQ,S-diene-l ,4diimine 3.0 38 6.5

+ Commercially available material.

EXAMPLE 2 20 EXAMPLE 3 The effect of mixtures of a p-phenylenediamine and the present compounds on the processing safety was investigated using the natural rubber formulation (la) of Example 1. The results of the Mooney Scorch Tests (Table 3) show that processing safety is improved in the presence of the p-phenylenediamine, whilst still retaining the antiozonant properties of the rubber vulcanizate.

TABLE 2 Analysis of Rheometer Traces Formulation (la). Rheorneter are i 1, range 50. temp. 140"C Scorch intercept (a) Torque Time (bl Cure Level Minimum time L+2 of max slope rise to L rate Additive (pphr) torque L (min.) on L (min.) above (M-L) (b-a) minimum (min.) (min.)

None 14 7 8 60.5 27.5 19.5 N.N-Di(1 eth l-3-methylpentyl l) cycllb-hexa-Zj- 5 8 l L5 l9 63.6 36.5 19.5 diene-1.4-diimine N-( l,3-Dimethylbutyl)-N-phenylcyclohexa-2,5-tliene- 1 ,4-diimine 2 7 9 4 67.5 32 18 Table 3 Mooney Scorch Antiozonant Properties at C (Critical Strain Additive Cone" Minimum t 25 pphm of ozone at (pphr) viscosity (min. min. 30C) None 28 22 4 3-5 N,N'-Dicyclohexyl-p-phenylenediamine 2 21 13.5 2 5-10 N,N'-Dicyclohexyl-p-phcnylencdiaminc 4 21 1 1 2 N.N-Dicyclohexyl p-phenylenediamine 2 19.5 21 5 10-15 N-Phenyl-4-iminocyclohexa-2,5-dien-l-onc 2 N,N-Dieyclohexyl-p-phenylenediaminc 2 N ,N'-Di( 1-ethyl-3-methylpentyl )cyclohexa- 17 18.5 3 20 2,5-diene-1.4-diimine 2 N,N -Di( 1 ethyl-3-methylpentyl )cyelohexa- 2,5-dienel ,4-diimine 5 19 52 25 N,N'-Dieyelohexyl-p-phenylenediamine 4 13.5 19.5 2.5 20 N-Phenyl-4-iminocyclohexa-2.5-dien-l-one 2 N.N"Dicyclohcxyl-p-phenylenediamine 4 20 15 19.5 3 N-Diphenylcyclohexa-Zj-dicne-1.4-diimine 3 EXAMPLE 4 The NR formulation (Ia) of Example I (Mooney Scorch, t,, at 120C, 22 min) was worked on the mill for 45 min. at l lOC. Various levels of two retarders were added to portions of the worked mix and the Mooney Scorch measured (Table 4). The processing safety is completely recovered by less than 2 pphr of the additives.

EXAMPLE 6 The processing safety of some cyclohexa 2,4-diene derivatives (Table 6) was examined in comparison with the corresponding p-aminophenol or pphenylenediamine in the following Styrene-butadiene copolymer formulation.

Table 4 Recovery of Scorched Stock Mooney Scorch at 120C Additive Level Minimum T (min) pphr Viscosity None 15.5 5.5 N,N Diphenylcyclohexa- Formulation ll Parts by Weight 2,5-diene-l ,4-diimine I 15 15.5 Styrene-butadiene copolymer 100 2.5 ]l 5 27 Zinc oxide 5 5.0 H5 34 Stearic acid 2 N,-( l,3-dimethylbutyl)- Process oil 5 N-phenylcycl0hexa-2,5- l l2 l8 HAF black dienel ,4-diimine CBS l.5 2.5 9 3l S 1.5 5.0 12 37 Additive Varies TABLE 6 Improved processing safety on an SBR mix Mooney Scorch at 130C Level Minimum pphr Viscosity t t min min None 47 26 4 N-PhenyI-4-aminophenol 2 41 17 5.5 N-Phcnyl-4-iminocyclohcXa-2.5-dicn-lone 2.5 38.5 26 24 N.N'-Diphcnyl p-phenylenediamine 2 M5 23 5 N.N'-Diphcnylcyclohexa-2,5-diene-l ,4-diiminc 2.5 40 26.5 I l N-lsopropyl-N'-phenyl-p-phcnylcnediamine 2 44 20 3.5 N-lsopropyl-N"phenylcyclohexa-l,S-dienc-l,4-diiminc 2.5 42 22 6 N,N' Di( l-ethyl-3-methylpcntyl)p-phenylencdiaminc 2 42 15 3.5 N,N'-Di( l-ethyl-3-methylpentyl) cycloheXa-2,5- dicnc-l ,4-diimine 2.5 44 26 3 EXAMPLE 5 EXAMPLE 7 Table 5 Although conventional PVls improve the processing safety of rubber mixes they do not affect the antidegra- 5O dant properties of the vulcanizates. The reaction of these new PVls with MBT during vulcanization produces good antidegradants. Tables 7 and 8 compare the Processing safety of polyisoprene mixes Mooney Scorch at C Compound Level pphr Minimum t,(min) t -,(min) Viscosity No Additive 47 22.5 6 N-phen yl-4 im inocyclohexa-2,S-diene- I -one (2.5) 35 43 17 N,N'-Diphenylcyclohcxa-Z,5-dienc-1,4-diiminc (2.5) 23.5 43 l! N,N'-Di( lclhyl-3methylpcntyl)cyclohcxa 2.5-diencl ,4-diiminc (2.5) 33 37 6 N-( l ,3-DimcthylhutyU-N '-phenylcyclohcxa 2.5-dicnc l.4-diimine (2.5) 31 38 7 antidegradant properties of vulcanizates (Formulation la) compounded with a conventional PVl, a cyclohexa- 2,5-diene-l ,4-diimine and the reaction product of a cyclohexa-2,5-diene-l ,4-diimine and MBT (N- Cyclohexyl-N '-pheny|-3-(benzothiazole-2 '-thiyl)-pphenylenediamine (l R=cyclohexyl; R'=phenyl).

Table 7 Air Oven Ageing Resistance at IOUC Tensile properties Cone" Unagcd Aged 2 days Aged 3 days Aged 5 days Additive pphr TS M 100 EB TS M l 00 EB TS M l 00 EB TS M [00 EB None 28.5 23 SH] 7.8 2.3 220 5.4 1.) 215 3.8 28 I Santogard PVI 1.0 27.9 2.4 53l 7.3 L9 283 4.8 [.71 225 3.7 2.8 127 N-( 1.3-Dimethylhutyl N-phcnylcyclohcxa-2,5- diene-l,4-diimine 2.0 3l.l 2.4 560 22.2 3.3 425 l5.4 3.l 330 6.8 3.3 2l5 N,N"Di( lethyl-3-mcthylpentyl]cyclohexa-2,5- diene-l.4-diiminc 2.0 3L2 2.4 540 21.4 3.9 375 l7.4 3.9 3l5 7.9 4.3 175 N-Cyclohcxyl-N phenyl- 3-(benzothiaz0le-2 -thiyl]p-phcnylenediuminc 3.0 28.4 2.5 52l l8.4 3.5 3l7 14.3 3.5 273 7.7 3.77 178 T5 Tensile strength (MNm' M100 Modulus ui lllUZ extension lMN.m

E8 C4 Elongation at break.

Table 8 Fatigue and Abrasion Resistance Additive Cone" Akron Abrasion Tcnsion Fatigue Life to break) pphr Volume loss at IS slip Extension Cycle at frequency angle of 5 Hz mm ISOO revs.

U 75% [)l(l0% 047.5% None 47 145 75 N-( l 3-Dimethylbutyl )-N phenylcyclohcxa-2,5-dicnel ,4- diimine l.() 22 495 265 l 10 N,N'-Di( l-cthyl-3-methylpentyl)cyclohcxa-2,5- diene-l ,4-diiminc 2.0 24 345 200 50 Standard PVl 57 w l i agent, and a thiazole vulcanization accelerator, charac- 1. A method of inhibiting the premature vulcanization of a rubber vulcanizable by heating with a sulphur vulcanizing agent and a thiazole vulcanization accelerator, which method comprises incorporating into the rubber, in addition to the sulphur vulcanizing agent and the thiazole vulcanization accelerator, a cyclohexa-2,5- diene derivative having the formula terised by containing, as a premature vulcanization inhibitor, a cyclohexa-2,5-diene derivative having the formula where X is O or NR and R and R may be the same or different and are inert alkyl, cycloalkyl or aryl groups.

6. A composition as claimed in claim 5 wherein each of R and R is an alkyl or cycloalkyl group containing from 1 to 10 carbon atoms or a monocyclic aryl group.

7. A composition as claimed in claim 5 wherein the cyclohexa-2,5-diene derivative is used in an amount of from 0.1 to ID parts by weight per parts of dry rubher.

it i i t #I 

1. A METHOD OF INHIBITING THE PREMATURE VULCANIZATION OF A RUBBER VULCANIZABLE BY HEATING WITH A SULPHUR VULCANIZING AGENT AND A THIAZOLE VULCANIZATION ACCELERATOR, WHICH METHOD COMPRISES INCORPORATING INTO THE RUBBER IN ADDITION TO THE SULPHUR VULCANIZING AGENT AND THE THIAZOLE VULCANIZATION ACCELERATOR, A CYCLOHEXANE-2,5-DIENE DERIVATIVE HAVING THE FORMULA
 2. A method as claimed in claim 1, wherein each of R and R'' is an alkyl or cycloalkyl group containing from 1 to 10 carbon atoms or a monocyclic aryl group.
 3. A method as claimed in claim 1, wherein the cyclohexa-2,5-diene derivative is used in an amount of from 0.1 to 10 parts by weight per 100 parts of dry rubber.
 4. A method as claimed in claim 1, including the step of processing the vulcanizable rubber mix by calendering or extrusion or forming it into a desired shape prior to vulcanization.
 5. A vulcanizable composition comprising a sulphur-vulcanizable rubber, a sulphur vulcanizing agent, and a thiazole vulcanization accelerator, characterised by containing, as a premature vulcanization inhibitor, a cyclohexa-2,5-diene derivative having the formula
 6. A composition as claimed in claim 5 wherein each of R and R'' is an alkyl or cycloalkyl group containing from 1 to 10 carbon atoms or a monocyclic aryl group.
 7. A composition as claimed in claim 5 wherein the cyclohexa-2, 5-diene derivative is used in an amount of from 0.1 to 10 parts by weight per 100 parts of dry rubber. 